1. Field of the Invention
This invention relates generally to communication between physical objects, item and inventory identification, tracking and management; and more particularly to a system for providing information concerning the identity or status of inventory using a plurality of labels enabled for communication with a computerized inventory management system so that label location and status are tracked continuously from a remote location.
2. Description of the Prior Art
Radio Frequency Identification (RFID) systems that utilize RF signals as means of communication between transponders, normally tags or similar modules, and interrogators, normally called readers, have been proposed by numerous prior art workers; see e.g. U.S. Pat. Nos. 3,299,424 and 3,689,885.
RFID tags in their simplest form comprise an ID, normally in a digital binary form, that is modulating an energy carrier signal such as an electromagnetic or acoustic wave, and is propagated by the tag as described in U.S. Pat. No. 3,713,148. The modulated carrier signal may be other energy carriers, e.g. acoustic waves or light beams.
Radio communication between a transponder tag and an interrogator can be carried out in two substantially different approaches. The first approach involves use of a circuitry in the tag. When exposed to the electromagnetic or acoustic field generated by the reader, the tag antenna comes into oscillation or similarly can couple with the reader field. The tag can use this coupling effect, which manifests itself as an alteration of the original field generated by the reader to present its ID or data. This coupling can be used to link the reader and the tag together. When radio frequency (RF) is used, this coupling can be magnetic coupling (near-field electromagnetic coupling) or backscattering (far-field electromagnetic coupling). The electro magnetic field generated by a reader's antenna induces a current in the tag whose receiver is tuned to the frequency of the field. When the wavelength of the frequency range used greatly exceeds the distance between the reader's antenna and the tag, the electromagnetic field may be treated as an alternating magnetic field and be considered as a transformer with one coil (antenna) located on the reader and the other coil (antenna) located on the tag. Magnetic coupling is commonly deployed in LF (Low Frequency) and HF (High Frequency) bands. The most popular frequencies for magnetic coupling are 135 kHz and 13.56 MHz. When the far-field electromagnetic coupling is deployed, the tag modulates its data back on to the electromagnetic field of the reader by changing the impedance of its own receiving antenna. This change of the impedance causes the tag antenna to effectively act as a reflector. Changes in the antenna impedance effectively reflect some of the electromagnetic energy back to the reader; the reader can then perceive the pattern of the modulation in the reflection. This phenomenon is called backscattering. Under these circumstances the reader can sense the presence of the tag, transmit data and receive the response back from the tag by demodulating the data that the tag has modulated into the field pattern caused by magnetic coupling or backscattering; see e.g. U.S. Pat. Nos. 3,516,575 and 3,541,995.
The second approach is to have a set-up like the one in conventional RF communication. The readers transmit signals that are received by the tags and the tags transmit signals, by means of a transmitter stage, that can be detected and decoded by the readers. With this approach, the structure of the signal transmitted by the tag is inherently independent of the signal received by it. The tag can receive information from the reader in one band and transmit it in a completely unrelated band and with a different signal structure and technology.
There are variations of the first approach that use backscattering in a band whose center frequency is an integer multiple or fraction of the center frequency of the received signal, but this flexibility is limited to similar simple techniques. There are also other approaches using Surface Acoustic Wave, Acoustomagnetic and electrical coupling as means of responding to the reader. However, these approaches can all be classified in the same category of devices that generate a reaction to the original field created by the reader and manipulate the same through this reaction.
In addition to the mechanism needed to modulate and propagate the response of the tag to the reader, other functional units in the tag, require power. One such unit is the logic engine that processes and transports the stored ID or data. Power can be provided by a source of energy that is integrated with the tag, e.g. a capacitor, a battery or an accumulator of some kind. It can also be generated by other means, e.g. by capturing the electromagnetic energy propagated by the reader or similar sources of energy carrying signals. The former category of tags is called active and the latter is called passive; the hybrids constitute the category of semi-active tags. In the case of RF signals, the process of power recovery from the incident signal requires a circuitry that can convert electromagnetic energy to such current and voltage levels that can satisfy the power needs of the tag.
In the first approach, the tag can be a completely passive element in that it can be powered up by rectifying the incident signal and since it is merely reflecting back the incident continuous wave (CW), it does not need to take on the power-consuming task of generating a CW as a signal carrier for transmission. The passive tag responds by presenting its ID or other data through manipulating the incident signal that is in turn sensed by the reader monitoring the frequency band in which the particular modulation is expected.
In the second approach, transmitting the data back to the reader requires power like any other RF transmission, because a carrier needs to be generated and depending in part on the frequency of the carrier, the complexity of the modulation scheme and the required power output. The amount of required power can easily fall outside what can be recovered from the incident signal. Therefore, the second approach is often only applicable to the category of active or semi-active transponders.
Magnetic coupling works only at very short distances. Backscattering relies on small signal reflections that only offer a limited range and a low bandwidth for data exchange between the tag and the reader. Tags made with this approach are inexpensive to manufacture. Their transmission stages are active and their active control and data processing stages are uncomplicated and utilize low power. At short range, they can supply their needed power by capturing electromagnetic energy through simple and affordable power rectification circuitry located on the tag.
Regardless of whether the tag acts as an active transmitter or backscatters passively, communication between a tag and a reader is performed in specific regulated frequency bands. The amount of output power in each band is regulated to protect other devices and bands against interference and saturation. These bands are normally narrow bands in LF (Low Frequency 0.03 MHz to 0.3 MHz), HF (High Frequency 3 MHz to 30 MHz), UHF (Ultra High Frequency (300 MHz to 3000 MHz) and Microwave portions of the RF spectrum.
Generally, there are a number of problems associated with currently available narrowband RFID technologies. Low data rate and lack of noise immunity, limit an item-level tagging and high simultaneous number of interrogations by the reader. The tags are nearly useless in presence of metallic objects, conductive materials, liquids and in general such material that can cause absorption of the RF energy or detuning of the signal. Additional RF problems such as path-fading and multipath interference are frequently encountered by continuous wave (CW) technologies
Attempts to remedy the above problems have thus far resulted in additional complexity that increases system costs and power requirements. These issues are mostly addressed by deploying a radio technology that spreads its signal over an extremely wide frequency band. Such technology is conventionally known as Ultra Wide Band (UWB) radio and is described in U.S. Pat. No. 5,677,927. According to the Federal Communications Commission (FCC) definition, a UWB signal has a fractional bandwidth greater than or equal to 20% or a total bandwidth of 500 MHz or more regardless of the fractional bandwidth. The fractional bandwidth percentage is defined as:
      B    f    =            2      ⁢                                                  f              h                        -                          f              l                                                          f              h                        +                          f              l                                      ·        100            ⁢      %        ≥          20      ⁢      %      where Bf is the fractional bandwidth, fh and fl are the highest and lowest −10 dB frequencies of the signal spectrum. The fact that these extremely wide bands will traverse over narrow bands of frequency dedicated to different operations, puts severe constraints on the power output of a UWB transmitter that currently is at or below that of unintentional electromagnetic radiations from electric devices. The definitions set by FCC aim at drawing guidelines applicable to regulating the use of the technology rather than defining the qualitative differences that distinguish UWB from the conventional radio technology. Furthermore, the FCC may change the quantitative criteria that constitute the definition of UWB radio; it may even make substantial changes to the definition. Despite the plasticity in the definition, the only reliable reference is the one set by the FCC, even if it is subject to future changes. The FCC definitions and regulations can be found in the First Report and Order: Revision of Part 15 of the Commission's Rules Regarding Ultra-Wideband Transmission. Federal Communications Commission, Feb. 14, 2002. ET Docket 98–153.
Before its current nomination, UWB Radio was intermittently called Impulse Radio, Carrierless Radio or Baseband Radio. In practice of UWB radio technology, very short duration impulses were deployed in a time domain that would create an extremely wide bandwidth in the frequency domain. That is, UWB was principally an Impulse radio that was per definition carrierless as opposed to conventional RF technology that uses a carrier frequency as means of propagating information. This specific technology is called Ultra Wide Band Impulse Radio (UWB-IR). The pulse shape was evolved to have optimal behavior in terms of antenna and receiver design as well as spectral efficiency. As a consequence of regulation by the FCC, approaches other than pure Impulse Radio have been proposed to be deployed within the regulated spectrum under the notion of UWB. The appeal of the newly introduced large spectrum segment caused new thinking in the domain of traditional radio design. Shannon's theorem, describing the relationship between the bandwidth and data rate, would yield equal benefits to a conventional radio technology that could be fitted to the ultra wide bandwidth. The theorem formulated as
  C  =      B    ⁢                  ⁢                  log        2            (              1        +                  S          N                    )      
describes how capacity (C) tends to remain high in spite of a very low signal to noise ratio (S/N) as long as bandwidth (B) remains high. Traditional radio technologies adapted to the wider bandwidth will not however yield other qualitatively crucial features of Impulse radio, such as low power and low complexity, resilience to multipath fading and unmatched location determination potential. A conventional radio technology will remain a Frequency Domain technology even at an ultra wide bandwidth as opposed to an Impulse Radio that is a Time Domain technology.
Narrowband RFID techniques are invented and utilized across a broad range of applications. UWB-IR radio communication has also been applied to RFID in several embodiments for different applications; see e.g. U.S. Pat. No. 6,550,674. The embodiments involve larger tags that deploy internal batteries and complex transceivers.
As a consequence there remains a need in the art for a communication system with tags that (i) packs a large amount of data such as inventory status, part location and the like; and (ii) is capable of simultaneously communicating with a massive number of transponders to enable item-level tagging; and (iii) consumes very low power, enabling remote operation, while at the same time being resistant to noise levels present in common industrial working environments.